1. Field of the Invention
The present invention relates to an anti-skid brake system for an automotive vehicle.
2. Description of the Related Art
Recently, an increasing number of automotive vehicles are loaded with an anti-skid brake system (usually called the "ABS system") which is arranged to prevent the wheel from being locked at the moment of braking. In order to operate the anti-skid brake system, basically, it is required to determine a slip value of the wheel that is incipiently locked and the slip value is determined on the basis of a speed of the vehicle body and speeds of the wheels.
Generally speaking, as disclosed in Japanese Patent Laid-Open Publication (kokai) No. 196,853/1986, the slip value is determined on the basis of a vehicle body speed assumed from the wheel speeds. Further, the fastest wheel speed among the four wheel speeds is employed as the assumed vehicle body speed.
Recently, there has been desired such an anti-skid brake system as capable of shortening the braking distance as much as possible. At this end, the assumed vehicle body speed is required to be determined as a value that is closest to an actual vehicle body speed. Even if the actual vehicle body speed is identical to the wheel speed, it is determined that the wheel has the tendency that the wheel is more incipiently locked as the assumed vehicle body speed becomes larger, and the anti-skid brake system is controlled more frequently by decreasing the braking liquid pressure, resulting in an increase of a total quantity of control and leading to eventually prolonging the braking distance.
The automotive vehicle is often accelerated before the start of control of the anti-skid brake system and the speed of the driven wheel may become considerably higher than the actual vehicle body speed. Hence, if the assumed vehicle body speed is determined on the basis of the speed of the driven wheel, the timing at which the control of the anti-skid brake system is performed may be put forward or the quantity of control at the moment of starting the control of the anti-skid brake system may become larger, thereby causing the problem with shortening the braking distance.